Efficacy and safety of combined oral sucrose and nonnutritive sucking in pain management for infants: A systematic review and meta-analysis

Background Pain management is currently important in neonatal intensive care unit (NICU). The superiority in pain relief of the combined oral sucrose (OS) and nonnutritive sucking (NNS) to other single intervention has not been well established. The administration of sucrose has been considered to potentially induce adverse events, which has been controversial. This study aims to investigate the combined effects and safety in comparison with other single intervention methods, including NNS, OS alone, breast milk and oral glucose. Methods We searched databases including Medline (via Pubmed), Embase (via Ovid), web of science, and Cochrane Library for randomized controlled trials from Jan 1, 2000 to Mar 31, 2021. The data were analyzed in the meta-analysis using Review manager Version 5.3. Pain score was the primary outcome in this meta-analysis. The adverse events were assessed qualitatively. Results A total of 16 studies were eligible in the meta-analysis. The results demonstrated a significant reduction in pain score in the NNS+OS group compared with NNS alone (SMD = -1.69, 95%CI, -1.69,-0.65) or sucrose alone (SMD = -1.39, 95% CI, -2.21,-0.57) during the painful procedures. When compared NNS+OS with breast milk, no significant difference was detected (SMD = -0.19, 95% CI: -0.5, 0.11). Conclusion The combined effects of NNS and OS might be superior to other single intervention method. However, the effects might be mild for moderate-to-severe pain.


Introduction
Newborns, especially preterm infants, are frequently subjected to painful procedures [1], repeated painful and stressful stimuli may develop clinical, physiologic and psychologic sequelae in the short or long term [2][3][4]. In some developing countries, most infants were not provided any analgesic intervention during painful procedures [5,6]. Even in a developed country like Canada, analgesic interventions were not offered in almost half of painful procedures, according to an epidemiology study [7]. In addition, it remains unclear whether the administration of anesthetics is safe [8]; thus, selective use of pharmacological treatment is recommended, such as opioids, non-opioids, and other anesthetics [9]. On the other hand, nonpharmacological methods have been developed in recent years to help reduce multiple bedside interruptions and have been considered safe and effective in pain management [10,11]. An updated research revealed the use rate of nonpharmacologic interventions tended to be higher than pharmacologic interventions [12].
Combined therapy was assumed to be more efficient in pain relief than single interventions and has been investigated frequently in recent studies [13][14][15][16]. For example, combined effects of music and touch [17], sucking, breast milk and tucking [18], music and sucrose [19], oral sucrose (OS) and nonnutritive sucking (NNS) [20,21], etc. were evaluated in clinical trials. Among which the combined effects of NNS and sucrose were tested by numerous prospective randomized trials. It has been common knowledge that OS and NNS could separately alleviated pain in neonates. Previous reviews or meta-analyses have preliminarily evaluated the combined effects of NNS and sucrose [10,13]. However, they may have neglected the variations in effectiveness and safety under different painful procedures. The superiority of the combination to various single interventions was also barely covered [8]. Therefore, this systematic review and meta-analysis aim to fill the gap by evaluating the efficacy of combining OS and NNS in different scenarios of comparison and exploring the safety of this intervention method preliminarily.

Literature search and screen
We conducted a systematic search from the following databases for English language articles: Medline (via PubMed), Embase (via Ovid), Cochrane Library, Web of Science. The search terms were based on three domains "newborns", "sucrose and non-nutritive sucking" and "procedural pain". The outcome-related terms were not restricted. Randomized controlled trials were selected using database-specific limiters. The time span of publication years was limited from Jan 1, 2000 to Mar 31, 2021. The search strategies for each database were provided (S1 Table). In addition to the electronic searches, Google Scholar and references in literature were also manually searched for potential suitable studies. The unpublished studies were not considered. The language restriction was English.
The literature research and screening procedures afterwards were performed independently by two researchers. The final eligible studies were cross-checked. If there were any disagreements, a third author made judgments.

Inclusion criteria.
(1) The study was a randomized controlled trial. (2) The paticipants were preterm or full term neonates without severe illness. (3) The randomized groups in the trial should at least contain an intervention group applied with OS combined with NNS. (4) Conference abstracts eligible for the inclusion criteria above were also included.

Exclusion criteria.
(1) Results of pain score were presented as categorical outcome.
(2) The sample size, mean and standard deviation were partly provided and could not be estimated from other statistics such as mean difference and p-value or median, range and interquartile range. (3) Non-English literature was excluded. (4) The outcomes of no interest, e.g., the occurrence of startle, jerk or tremor were excluded.

Literature quality evaluation
We conducted literature quality evaluation according to the Cochrane Risk of Bias Tool (RoB), version 5.2 [22]. The risk of bias was assessed on a 3 level scale:"low", "high" and "unclear" risk based on the following seven domains: random sequence generation, allocation concealment, blinding of participants and personnel, blinding of outcome assessment, incomplete outcome data, selective reporting, intention-to-treat analysis, and a completeness of follow-up.

Data extraction
Data extraction was performed by two reviewers independently. The summary of studies was established from the following information: first author, year of publication, country, study design, gestational age, pain procedure, sample size and specific intervention descriptions in each group and outcome.
We implemented the meta-analysis on pain score measured in different painful procedures, including Premature Infant Pain Profile(PIPP) [23], the premature infant pain profile-revised (PIPP-R) [24], Neonatal Infant Pain Scale (NIPS) [25], the Neonatal Facial Coding System (NFCS) [26] Neonatal Pain Agitation and Sedation Scale (N-PASS) [27]. The results of pain score were required to be presented as continuous variables. The Means and standard deviations (SDs) were extracted directly from the original data if available. Otherwise, they were estimated using Hozo et al.'s Method or Bland's Method under the scenario where median, IQR or ranges were available [28][29][30]. Calculations using p-value, confidence interval and mean difference were performed following the instructions in Cochrane handbook 5.1 [22]. Statistics were estimated from figures if applicable when no digits were provided. Missing data were not imputed. For cross-over designs trials, paired analysis was performed to standardize the means and SDs to account for the within-subject correlation [31]. If no information about within-subject correlation was provided, we decided to assume the correlation to be 0 which was a conservative way since there were no similar data to be referred in the included studies.
For evaluating the effects at different timepoints, we extracted the outcome data in 2 phases, (1) during the procedure phase (during or within the first minute immediately after the painful procedure), and (2) the recovery phase (1 to 5 min after the procedure). Pairwise comparisons were performed between the intervention group of interest, i.e., sucrose and NNS group and any other relevant groups including NNS (NNS+water)/sucrose alone, breastfeeding, breast milk, glucose or routine care group. In crossover trials, the overall effects were extracted instead of looking at a single sequence.

Statistical analysis
Results of pain score were presented as standardized mean difference (SMD) if holding different scales and 95% CI considering variety among pain measurements. If studies shared the same measurement then mean difference (MD) could be presented. A p-value <0.05 was considered as significant. Statistical heterogeneity was assessed using I 2 tests. Subgroup analysis or random-effects model applied when the statistical heterogeneity was high (I 2 >50%) [22]. Otherwise, a fixed-effects model was conducted. Sensitivity analyses were conducted to assess the stability and validity of results. Factors were considered when removing certain studies, including article types, outcome measurements (PIPP or non-PIPP), target population (preterm/full term), etc. Publication bias was evaluated by Egger's Test and funnel plots. All the statistical analysis was performed using Review Manager 5.3 [32]. Egger's Test was performed using SAS version 9.4. Besides, the safety of the intervention was qualitatively evaluated based on the occurrence and types of adverse events.

Outline of eligible studies
A total of 232 records were identified using our searching strategy from databases and two from the references in a previous review [10]. After removing duplicates, 79 records were obtained and 63 of them were screened in the first stage by looking through abstracts and titles. The first round of screening led to 31 records for the following full-text review. Finally, 16 studies [33][34][35][36][37][38][39][40][42][43][44][45][46][47][48][49] were included in our systematic review and meta-analysis. The screening process was described as the PRISMA flow diagram (Fig 1).

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painful procedures including heel stick (n = 8), retinopathy of prematurity (ROP) screening (n = 5), venipuncture (n = 2) and wound dressing (n = 1). PIPP score was mostly widely used for pain assessment (n = 10) while PIPP-R(n = 1), NIPS (n = 3), NFCS (n = 1) and N-pass score(n = 1) were also applied in other studies. Characteristics of eligible studies were outlined in Table 1. Fig 2A, 2B demonstrated the risk of bias assessment for each study. Prospective, Randomised, placebo controlled study preterm ROP screening group 1: n = 10 < 32 weeks' PMA Group 1: 1 ml sterile water given by mouth using a syringe PIPP scores during eye examination group 2: n = 10 Group 2: 1 ml sucrose 33% given by mouth using a syringe group 3: n = 9 Group 3: 1 ml sterile water given by mouth using a syringe and pacifier put into the mouth group 4: n = 11 Group 4: 1 ml sucrose 33% given by mouth using a syringe and pacifier put into the mouth Collados,2018, Spain [36] Multicentre randomised, noninferiority, crossover trial preterm venipuncture EBM-sucrose sequence: n = 33 gestational age of less than 37 weeks at birth and weigh less than 2,500 grams.
In group one, the neonate received EBM during the first venipuncture that was included in the study and 24% sucrose during the second venipuncture. The process was reversed for group two.
PIPP, the duration of crying, oxygen saturation and heart rate sucrose-EBM sequence: n = 33

NNS+OS group versus NNS group.
A total of 11 studies with 677 participants explored the effect of OS combined with NNS compared with NNS alone during the painful procedures, most of which reported PIPP score, while one study reported NIPS [37] and one reported N-pass score [38]. In both heel-stick and ROP subgroups, a significant effect was observed under the random-effects model, with the standard mean difference being -1.59 (95%CI: -2.49, -0.68) and -1.05(95%CI: -1.56, -0.55), respectively. However, there was no significant difference in the wound dressing subgroup (Fig 3). A sensitivity analysis was conducted based on two aspects: removing conference abstracts and removing articles using non-PIPP measurements. The results remained significant after two conference abstracts were removed in the ROP group [33,35]. Besides, unifying the measurements by removing two studies reporting NIPS or N-pass score did not change the results or reduce heterogeneity neither. The analysis showed no effect in four studies involving term infants regardless of the type of procedures (p = 0.16) with SMD: -0.81 (95%CI: -1.94, 0.33) [37,[39][40][41].
Fewer studies provided information in the recovery phase. The available 4 studies showed the combined sucrose and NNS did not provide a better effect in relieving pain in both heel stick and wound dressing groups (Fig 4). However, this finding was not robust in the heel stick subgroup due to the reverse result (MD = -3.23, 95%CI: -4.56, -1.89, P<0.001) after removing Mitchell's study in the sensitivity analysis [39].

NNS+OS versus breastfeeding or breast milk. The comparison between OS+NNS
and breast milk showed no difference in effects for pain relief despite the heterogeneity in study design, intervention methods, pain procedures, outcome assessment and target

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population between the two studies (Fig 8). In Benoit's study [47], full term infants underwent heel stick and were direct breastfed, whose pain was assessed by PIPP-R. In contrast, 66 preterm infants underwent venipuncture in two sequences. During the controlling period, infants were given breast milk via a pacifier, whose pain was assessed by PIPP in Collados's study [36].

NNS+OS versus glucose.
Only one study enrolling 66 term newborns set glucose +NNS as the control group [48]. The result demonstrated a significant difference (p < 0.05) in both two phases. NIPS scores were significantly lower in the NNS+sucrose group (range 1-2 and median 0) compared with the NNS+glucose group (range 5-7 and median 6) during venipuncture. Fig  9A, 9B). The results of Egger's test indicated no publication bias in both scenarios. The bias

Adverse events
Among the included studies, ten trials [36-38, 40, 43-45, 47-49] reported the occurrence of adverse events. Five trials observed that the occurrence rate of adverse events was zero in all treatment groups [36,37,43,47,48]. Oxygen desaturation was the most common adverse  (Table 2). Three trials reported heart rate and oxygen saturation as mean (SD) as a secondary outcome (Table 3) [34,44,46]. They all observed a significant effect of NNS+sucrose on oxygen saturation and heart rate except a non-significant result in Asmerom's for heart rate.

Discussion
Pain management has been well established in recent years, especially in procedures involving skin punctures [12,50]. However, there is not a practical guideline on how to reduce pain and stress in a prolonged procedure such as ROP and wound treatment [51]. Therefore, this systematic review and meta-analysis aims to determine the effect of combined NNS and sucrose intervention in different types of painful procedures.
Overall, the combined interventions of NNS and sucrose showed the superiority in relieving mild pain during the painful procedures compared to applying NNS alone or sucrose alone or standard care. This has been confirmed by previous reviews [10,13]. Non-nutritive sucking was believed to be associated with antinociceptive mechanisms and sucrose appears to enhance the effect of NNS, leading to an increase of endogenous endorphins [9,52]. However, the mechanism of the combined effects is not completely clear [53].
On the other hand, the superiority is not significant when compared with direct breastfeeding or giving breast milk via a pacifier [36,47]. The similarity between sucking through a pacifier and breastfeeding might explain this result [52]. Besides, swaddling and a blanket used in the two trials helped establish a simulated environment as breastfeeding, calming and comforting the babies. The similar effectiveness in healthy term neonates or stable late preterm neonates between sucrose/glucose administration and breast milk was also proposed in Shah's review [54].
ROP examinations and wound dressing have two points in common. Both procedures last for several minutes and tend to give giving stronger painful stimuli. The insertion of an eyelid speculum and scleral depression usually increases the intensity of the pain during ROP screening [55]. Although a significant effect was observed in ROP examinations in a preterm population, we found the means of PIPP score even in the combined NNS and sucrose group still around or higher than 12, qualitatively, indicating a moderate-to-severe level. Considering the preterm infants tend to have lower scores than term infants, such high scores indicated the finite effect of this combined intervention [56]. Besides, a non-significant effect was detected during wound dressing in Mandee's study where moderate-to-severe pain were often observed [37]. This might not be indicative because few studies researched on pain managements in wound dressing. However, previous studies found the effect of sucrose were time-dependent and could not last for a long time [10,41]. This might be able to explain the minor effect. More innovations are needed on non-pharmacological interventions for those intensive procedures. For example, a recent study recommended the physiological flexion position called ROP position and it was demonstrated more effective than the combination of sucrose and NNS [57]. The combination of multiple non-pharmacological methods, including non-nutritive sucking, oral breast milk, and facilitated tucking, music, etc. might also help enhance the effects of pain relief [17,18,58]. Investigations on more effective methods are warranted, especially for lowering moderate or even severe pain.
Few studies in our analysis reported the pain measurements in the recovery phase, which might lead to an unreliable result in the meta-analysis. Our analysis found the effect has vanished in heel stick procedure when comparing the combined interventions with NNS alone group during this period. This finding could be reversed by removing a study with nonsignificant result in sensitivity analysis [39]. In this trial, both intervention and control groups received a pacifier and facilitated tucking resulting a low PIPP and sucrose did not contribute much to reducing pain. In fact, facilitated tucking has been reported to be effective in pain management [59].
Subgroup analysis on sucrose administration was not performed in the meta-analysis because of the varieties in different trials in terms of volumes (from 0.1 mL to 2 mL), concentrations (range from 12% to 33%) and frequencies. However, Steven found no difference in effectiveness among doses by proposing the minimal effective dose of only 0.1 mL in relieving procedural pain [60]. Thus, the wide range of volumes of sucrose might not influence the comparison.
OS has been considered to cause adverse events such as desaturation and bradycardia, especially in preterm infants [10]. Our analysis indeed indicates the trend of oxygen saturation increasing while heart rate decreasing under intervention. However, we did not observe a higher occurrence of bradycardia, tachycardia or desaturations in groups with OS. Besides, adverse events indeed occurred more in preterm subjects. Trials with zero adverse events were mostly based on full-term infants. This might indicate the vulnerability of preterm infants attributes more to adverse events than OS. Nonetheless, the evidence might be influenced by reporting bias because some studies did not report any information on the safety during the intervention procedure.
Although the combined intervention of sucrose and NNS was considered effective in reducing pain among the studies included, sucrose was often administered 2 min prior to the painful procedure in these studies. This time interval was unjustified by Meesters in 2017 [61]. Besides, the recovery phase was neglected in half of the studies. The recovery from pain is important as it also affects the baby in a long-term way. We might improve the intervention methods to deal with moderate and severe pain and pay more attention to the recovery phase.
De Bernardo's trial found sucrose was more effective then oral glucose when both combined with a pacifier. This conflicts with a previous review which take glucose as an acceptable alternative to sucrose [62]. More studies could be conducted in this direction in order to expand the available and effective intervention methods.
There are three limitations in our analysis. Firstly, the heterogeneity is non-negligible in heel-stick subgroups but acceptable in other painful procedures. Heel-stick was involved in most studies. The variations in their protocols, such as population, the operating process, the pain measurements, or the sample size might interactively contribute to moderate to high heterogeneity. For example, some studies enrolled both preterm and term infants in the trial, while others enrolled purely preterm or full-term infants, which added the difficulty in explaining the effects on the certain population. Different scales and standards in pain measurements also influenced pooling data in quantitative analysis. We have conducted post hoc subgroup analyses to test the source of heterogeneity in terms of these factors. High heterogeneity might induce an unreliable conclusion even under a random-effects model. Results should be explained with caution. Secondly, pain score was the only one outcome to measure pain in this analysis and the different assessments were pooled. Those pain assessment methods are distinguishable from their target population, scales and evaluation items [63]. It was hard to unify because PIPP was the most common and other assessments accounted for no more than 25% of included studies. A sensitivity analysis was conducted to reduce the potential bias though. Other measurements including crying time were not investigated because of the limited number of studies. There were very few studies reporting crying time and comparing the combined intervention with sucrose alone or routine care. A single outcome of pain score might not be robust enough since different scales were used among studies. Finally, publication bias seems to exist in our analysis.

Conclusion
In conclusion, our systematic review and meta-analysis indicated the superiority of the combined intervention of sucrose and NNS than any single intervention except for breastfeeding. However, the effect appears to be mild in alleviating moderate-to-severe pain. More exploration and improvement of intervention were needed.